In this type of cellular network, the mobile terminal of a user usually measures the quality of its radio communication channel by virtue of pilots that are sent by the base station on the downlink, before using the uplink to return a channel quality indicator, CQI in English, to this base station.
Such an indicator CQI is representative of the signal to interference and noise ratio (SINR) perceived by the mobile terminal at the moment of its measurement.
FIG. 1 illustrates the typical method for establishing such a quality indicator in a cellular network comprising, purely by way of illustration, three base stations NB1 to NB3 that are respectively situated in three adjacent cells of this network.
In the cells associated with the base stations NB2 and NB3, which are adjacent to the first cell, these base stations NB2 and NB3 transmit signals of a nature to interfere with the communication, in the first cell, between the first transmitting device NB1 and a mobile terminal M.
In order to take account of this phenomenon of intercellular interference, the base station NB1 transmits, in the first cell associated therewith, a pilot signal SP, the characteristics of which are known to the mobile terminal M, on the downlink of a radio communication channel so that this mobile terminal M is able to take a measurement for the signal to interference and noise ratio SINR (by comparison with the known characteristics) and to deduce the corresponding indicator CQI therefrom.
The mobile terminal M is then able to reciprocally transmit, on the uplink of the radio communication channel to be evaluated, the indicator CQI to the base station NB1, so that the latter is aware of the transmission conditions on this channel, as measured by the mobile terminal M, and is able to adapt the transmission speed to these conditions.
This indicator CQI is used particularly by the base station NB1 in order to predict the signal to interference and noise ratio SINR to which the mobile terminal M will be subjected in the future, and thus to select the speed suited to this signal to interference and noise ratio SINR as predicted. In particular, the higher the predicted signal to interference and noise ratio SINR, the higher the selected speed, and vice versa.
The efficiency of this type of mechanism, called “link adaptation” (or “adaptive modulation and coding” in English) is dependent on the relevance of the indicator CQI returned to the base station.
If this indicator CQI is underestimated, the speed selected by the base station is lower than the optimum speed, which causes a loss of efficiency.
If this indicator CQI is overestimated, on the other hand, then the speed selected by the base station is higher than the optimum speed, which can cause an increase in the number of data retransmissions, and therefore an increase in the transmission delay.
This is a particular problem in the case of a user using a service that is sporadic (that is to say in which data packets are sent sporadically) and realtime-based (hence with a large delay constraint), as is the case with services concerning the transmission of voice, streaming or video games in a network.
The reason is that, when such a user, called “target user”, is in a cell of a cellular network, it is possible that another user, called “interfering user”, is likewise using a sporadic service in a cell that is adjacent to this cell, which can create sporadic interference that is not necessarily reflected by the indicator CQI returned to the base station of the cell of the target user.
In fact, in such a situation of sporadic interference, the returned indicator CQI can then represent a signal to interference and noise ratio SINR that differs enormously from the signal to interference and noise ratio SINR to which the data transmission is effectively subject.
Two scenarios may then arise:                in a first case, the returned indicator CQI proves to be too optimistic: if, at the moment of measurement of CQI by the terminal of the target user, the interfering cells are inactive, and they become active at the moment of the transmission of data, the returned indicator CQI will be too high in relation to the real situation, and therefore the speed used by the base station will be too high, which gives rise to a large number of retransmissions that greatly degrade the quality of the real time service of the target user.        in a second case, the returned indicator CQI proves to be too pessimistic: if, at the moment at which the terminal of the target user takes its CQI measurement, the interfering cells are active and they become inactive at the moment of the transmission of data, then the returned indicator CQI will be too low, and the speed selected by the base station will be lower than the optimum speed. The capacity of the cell (in terms of number of users supported) will then be reduced.        
This phenomenon is usually denoted by the term “CQI mismatch” in English and has been observed and measured in “3GPP LTE Downlink System Performance”, Farajidana et al., Global Telecommunications Conference, GLOBECOM 2009, where the notion of sporadic interfering user is denoted by the term “partial loading”.
It will be noted that this phenomenon of CQI mismatch can likewise exist even when the interfering traffic in the adjacent cells is not sporadic. By way of example, the CQI mismatch can be caused by a “flash light” effect corresponding to the situation in which an adjacent cell, although transmitting continuously (i.e. at “full loading”), saturates and greatly interferes, sporadically, with the target users, on account of a beam focusing process (“beam forming”).
As the CQI mismatch mainly affects the target users having a non-negligible interference to noise ratio, the users at the edge of a cell are most affected by this CQI mismatch.
A certain number of techniques have been developed in order to attempt to resolve this problem of CQI mismatch:
The base station can apply a fixed margin to the indicator CQI reported by the mobile terminal. The base station therefore uses a pessimistic CQI at the input of its link adaptation mechanism, so as to be robust toward interference variations. This technique has the disadvantage of applying the same margin to all mobiles, whatever their characteristic, and is therefore found to be less than optimum.                The base station can average several indicators CQI reported by the mobile terminal. The base station then uses an average indicator CQI at the input of its link adaptation mechanism so as to be robust toward interference variations. This technique continues to be less than optimum, especially insofar as the problem of optimizing the averaging window, per cell, is difficult to resolve.        It has been envisaged to resort to interference cancellation methods, which prove to be extremely complex, however, because, upon the reception of data, the mobile terminal needs to be capable of jointly demodulating the useful signal and the interfering signal in order to reject the interfering signal. This complexity is proportional to the number of interfering elements that need to be rejected.        It is likewise possible to use a technique in which all cells transmit all the time, and possibly padding bits in order to create stable interference. Such a technique is less than optimum because energy is needlessly expended for transmitting bits (padding) without meaning (padding), it is all the less optimum when the network has a low level of loading.        It is possible to resort to techniques of interference coordination between cells (ICIC), which likewise prove to be very complex in terms of planning, however, and do not prevent the persistence of overlap areas bringing about a reduction in the capacity of the cell.        It has likewise been envisaged to place an attenuator in the mobile terminal, directly downstream of the reception antenna and upstream of the analog/digital signal process chain. This attenuator attenuates the received signal before the noise owing to the analog/digital conversion is added. With a good attenuator, the interference becomes negligible in the face of the noise, and the system becomes less sensitive to interference variations.        
However, this solution has the disadvantage of causing attenuation of the useful signal received by the mobile terminal, which therefore systematically loses capacity and coverage. Moreover, the performance of the attenuator is dependent on the mobile manufacturers and therefore cannot be guaranteed.                Finally, a link adaptation technique as described in the article “Performance aspects of LTE uplink with variable load and bursty data traffic,” Rosa C. et al., Personal Indoor and Mobile Radio Communications (PIMRC), 2010, can be used.        
In this technique, the base station applies an offset to the CQI at the input of the link adaptation mechanism. The offset is controlled by a loop. If the base station receives a “NACK” message for a packet received by the mobile, the base station applies a reduced offset, and if it receives an “ACK” message, it applies an increased offset, the relationship between the steps of increasing and reducing the offset being dependent on the target “Block Error Rate” ratio.
This technique has the disadvantage that the base station can play with the offset only when data are transmitted. In point of fact, for a sporadic real-time service, the opportunities for data transmission are not very frequent. The BLER control loop therefore has difficulties converging.